Interferons (IFNs), first identified for their potent antiviral activity, can be divided into two major classes. Of these, type II or immune IFN (a.k.a. IFN-gamma) has earned much notoriety, but the type I IFNs (IFN-Is;e.g., IFN-alpha, IFN-beta, etc.) represent a much larger and complex family. Consistent with this, viruses have evolved numerous strategies to thwart IFN-I activity. More recently, IFN-Is have also achieved some celebrity with the recognition that they are the major effector cytokine secreted by plasmacvtoid dendritic cells (pDCs, a.k.a. "Natural IFN-I Producing Cells"). These IFN-Is then regulate aspects of both innate and adaptive immunity. There is also intriguing new evidence that IFN-Is and pDCs play an important role in the pathogenesis of Systemic Lupus Erythematosis (SLE). Like other cytokines, IFN-Is induce their potent activity through the induction of new genes. Characterization of the ability of IFN-alpha to rapidly induce genes led to the identification of Stat1 and Stat2, the first two STAT transcription factors. These STATs are recruited to the type I IFN receptor (IFNAR) by unknown mechanisms, whereupon they become activated (by tyrosine phosphorylation), dimerize, translocate to the nucleus and activate genes. In contrast, IFN-gamma transduces its signals solely through Stat1, albeit with differing kinetics. To determine the unique role Stat2 plays in the biological response to IFN-Is, Stat2 knockout mice were generated. These mice were highly susceptible to viral infection and partially unresponsive to IFN-Is. Unexpectedly, they exhibited tissue-specific differences in IFN-I stimulated Statl activation and a loss in the normal regulation of the Major Histocompatibility Complex class II (MHC-II). These observations highlight the important role type I IFNs play in regulating innate and adaptive immunity. To understand how IFN-Is mediate their many potent effects we propose to: 1. Determine how STATs are activated at the type I IFN receptor, including tissue specific differences. 2. Determine the unique role Stat2 exhibits in regulating MHC-II expression in macrophages 3. Explore the role SUMOylation may play in regulating the kinetics of IFN stimulated STATs.